“Dry” triboelectrostatic separation is widely used as an effective technique for separating different particulate solid components (“particles”) from a physical mixture entrained or carried in a driving fluid, such as air. Typical applications include the beneficiation of minerals, purification of foods, the recovery of valuable components from waste, and the sizing of particles in a particle mixture. This technology has gained widespread acceptance as providing a low cost, environmentally friendly technique, since it requires no chemicals or water and thus eliminates costly downstream de-watering and slime disposal applications required in wet separation processes.
Typically, electrostatic separation relies on the surface physical properties of the different particles and controlled flow conditions to effect beneficiation in an efficient and effective manner. Specifically, when two species of particles with different work functions contact one another, a charge transfer between the contact area results, such that one species may carry a positive charge and the other a negative charge (known as “contact charging”). This differential charge may also be achieved by “friction charging,” which results when the particles are forced to slide along or rub against a solid surface. The combined effects of these charges are together known as “triboelectrostatic charging” or “tribocharging” for short, and are together considered to play a key role in achieving particle separation.
FIG. 1 schematically illustrates a typical prior art triboelectrostatic separator S. The particles P in the mixture are fed into the separator S from a bin B, and are charged to a bipolar state in a metal tube T, mainly by friction charging. The particles P then pass through an electric field F, such that the species of particles having a particular charge is drawn from the mixture toward a corresponding electrode E1, E2. However, as a result of the inefficient charging resulting from the fact that not all particles make contact with the sidewalls of the tube T, weakly charged or charge-neutral particles may not be attracted and consequently simply pass through the separator S unaffected by the electric field F. While these “middling” particles (not shown in FIG. 1) may be separated during a second pass, this obviously decreases the efficiency of the separation operation. Increasing the feed rate of the particles P may allow for more passes in a shorter period, but a concomitant decrease in the separation efficiency per pass results because of the shorter residence time of the particles in the electric field F.
Accordingly, while the typical prior art separator S is effective for separating two particle species from a particle mixture, it should be appreciated that further improvements in separation effectiveness and operational efficiency are still possible. More specifically, a need exists for devices and methods that enhance the charging on the particles as well as the downstream separation to improve efficiency and potentially reduce the need for the number of passes required.